The use of mobile broadband (MBB) services is rapidly increasing in all regions of the world as a result of the ongoing transition from cellular telephony to MBB. Mobile data surpassed voice during December 2009 and yearly traffic increases in the order of 200% to 300% have been measured in real networks. This increase is predicted to continue.
The mobile operators therefore face the challenge of handling an immense traffic increase in their networks. The required solutions on the radio side will likely be based on a combination of a deployment of spectrally efficient technologies, a densification of existing deployments, and an introduction of additional spectrum bands. Some examples of spectrally efficient technologies are the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), which is a project to improve the Universal Mobile Telecommunication System (UMTS) standard, and later generations of High Speed Packet Access (HSPA). HSPA being a mobile telephony protocol that extends and improves the performance of existing UMTS protocols. There is a general agreement in industry, academia and regulatory bodies that more spectrum will be required for MBB services in the future.
One trend in radio research and regulation is based on the observation that many legacy systems provide an inefficient use of spectrum. A re-planning of such legacy systems could free up spectrum for licensed mobile use. Furthermore, significant efforts in research, standardization and regulation are spent on finding ways of getting higher spectrum utilization by means of secondary usage of said spectrum. A secondary user is in this context a user which is permitted to also use the spectrum e.g. for some other purpose than the legacy or primary system purpose, and that has well defined obligations to not cause harmful interference to the licensed, or primary, usage. The frequency range used by broadcast TV systems has become prime targets for secondary spectrum usage, and some regulatory bodies such as the Federal Communications Commission (FCC) in the US already have rules in place for secondary usage of TV bands. Other regulatory bodies have published suggestions for regulatory rules for consultation, such as the Electronic Communications Committee (ECC) within the European Conference of Postal and Telecommunications Administrations (CEPT) and the Ofcom in the UK. The main reason for the interest in the TV spectrum is that the spectrum is of high value to operators and that TV transmitters are stationary and predictable.
The presence of secondary users implies some sharing of spectrum bands between primary and secondary systems. Among the different possible sharing approaches, the interweave approach is the primary-secondary spectrum sharing approach which is probably the most commonly discussed in academia and elsewhere. In this approach the signals of the secondary systems are orthogonalized to the primary signals in time, frequency and/or spatial domain(s). This may be achieved, e.g., by letting the secondary systems communicate on time/frequency resources that are unused by primary systems. Another type of interweave is spatial/frequency orthogonalization where channels unused by the primary system at certain locations can be used by secondary systems.
TV white space is an example of this latter approach, schematically illustrated in FIG. 1. A primary TV transmitter 185a is operating on a particular channel N, and serves a primary service area 180 which is surrounded by a primary protection zone 181 in which no white space usage in that channel is allowed. Another primary TV transmitter 185b operating on channel with an associated service area 183 and protection zone 184, is also illustrated. Furthermore, a secondary LTE system 182 operating in a white space at channel N is illustrated. Secondary users in the LTE system may be referred to as white space devices (WSD). Channel N is thus a channel available for secondary usage by WSDs in the LIE system. Such a channel may also be referred to as a white space channel. The WSDs may be the LTE base stations, transmitting on channel N in the downlink, or user equipments (LIE) transmitting on channel N in the uplink. WSDs are thus devices that opportunistically use spectrum allocated for a primary system service on a secondary basis at locations—so called white spaces—where no primary system user is using the spectrum. The WSD is not allowed to cause harmful interference to the primary system service. Furthermore, the WSD is not protected from interference from any primary system service or user.
The fact that the white space is inside the primary protection zone 184 corresponding to channel N+k, may reduce the allowed transmit power for the WSD in the secondary system. Studies have shown that a big limitation for TV white space is the interference that the WSDs may cause to primary receivers, i.e. TV receivers, operating on channels other than that of the WSD. This problem will limit the WSDs output power, sometimes significantly, as will be explained hereinafter. As explained above with reference to FIG. 1, the WSD operating at channel N may be within the coverage area of TV transmitters operating on other frequency channels N+k, and is thus not allowed to operate on these other channels. It is in principle allowed to operate on channels used by other remote TV transmitters, as long as the WSD is located outside the primary protection zone of those transmitters, provided that the WSD limits its maximum transmit power below a value that would cause harmful interference to TV receivers. The allowed maximum WSD power is limited by the interference caused to TV receivers operating not only on the WSD channel, but also those operating on other channels. In the latter case the spatial distance between the WSD and a potentially interfered TV receiver can be very small, since the WSD could be located within the coverage area of the corresponding TV transmitter.
Since digital TV (DTV) receivers typically have limited frequency selectivity, as illustrated in FIG. 2, they are sensitive to WSDs operating even many channels away. FIG. 2 shows the protection ratio in dB as a function of the channel offset, FIGS. 3a-d show cumulative density functions that illustrate the amount of channels lost due to protection requirements on adjacent channels when using the white space rules proposed by the European CEPT SE-43 group, for different values of the Equivalent Isotropically Radiated Power (EIRP) of the WSD. FIGS. 3a and 3b illustrate the amount of allowed white space channels in Germany for two different types of WSDs when protecting TV receivers up to 10 channels away from the white space channel. The y-axis show the fraction of the surface of the country at which the number of channels on the x-axis or less are available. In FIG. 3a the amount of channels for a UE-like or portable WSD is shown, and in FIG. 3b the corresponding information is given for a base station-like or fixed WSD. From FIG. 3a it can be seen that a UE-like WSD with an EIRP level of 20 dBm have no available white space channels at all in over 30% of Germany's surface. In FIGS. 3c and 3d the corresponding results are shown when only TV receivers operating on the nearest adjacent channel to the potential white space channel are protected. Comparing these two sets of figures and the available white space shows that the amount of white space increases tremendously, in particular for the high power WSDs, when TV receivers operating several channels away from the white space channel are less protected from interference. If only TV receivers operating on the same channel as that of the WSD have to be protected, the amount of white space may increase even more.
It is clear from FIGS. 3a-d that the requirement on adjacent channel protection significantly limits the usable amount of white space channels, in particular for high WSD power levels. One way of removing these limits would be to regulate harder requirements for the digital TV receivers, e.g., such that only adjacent channel WSD interference affects the TV receivers. In this case the white space channel availability would be changed from that of FIGS. 3a-b to that of FIGS. 3c-d respectively. However, this would make TV receivers more expensive. Furthermore, it would require changing all TV receivers.